1. Field of the Invention
This invention relates to a method and apparatus for blending oxygen, in a piston-type ventilator which permits constant and accurate supply of supplemental oxygen to the user through a closed loop pressure support ventilation system. More particularly, it is concerned with a piston ventilator used to ventilate patients having difficulty respirating, which adjusts the amount of oxygen supplied both to the piston cylinder and to the patient circuit for ensuring a satisfactory oxygen content throughout the air in the circuit inhaled by the patient.
2. Description of the Prior Art
Ventilation systems are designed to supply air at a pressure greater than atmospheric pressure to assist in breathing, and may include systems which provide supplemental oxygen which is blended with air to enrich the gas which is inhaled. Such systems are typically employed for patients with respiratory ailments wherein the oxygen-enriched air is provided by blending bottled air or less often atmospheric air with supplemental oxygen in a controlled manner for each breath.
Another system for mixing the oxygen with air or another gas in a ventilation system in predetermined proportions involves the use of separate inlets into a pressure vessel up to respective first and second pressures is described in U.S. Pat. Nos. 4,022,234 and 4,023,587. The system shown therein operates in alternating withdrawal and mixing cycles. A feedback control of the rate of flow and pressure of breathing gas to a patient by an inspiration servounit is described in U.S. Pat. No. 3,741,208. U.S. Pat. No. 5,383,449 provides for control of oxygen concentration in a breathable gas by calculation of the mole ratios and pressure in the containment vessel, and by sequentially injecting oxygen and another gas to desired pressure values. These so-called batch mixing ventilators represent one system for patient ventilation.
While such systems are very useful in hospitals and other health care facilities, smaller and more confined devices not requiring connection to pressurized air are often more appropriate for home care. Piston and bellows types of ventilators allow delivery of a predetermined volume of breathing gas at a desired pressure responsive to the initiation of inspiratory efforts by a patient. Piston based ventilators can typically be made to be more compact than bellows based ventilators, but piston ventilators typically blend pressurized air and oxygen in a high pressure blender. The resultant mixture is then drawn by a piston through a valve that reduces the pressure of the mixture. Such systems typically do not permit the use of room air and pressurized oxygen, and can result in some risk of overpressurization in the event of failure of a high pressure gas delivery valve controlling introduction of one of the breathing gas components into the high pressure blender.
Another system for blending oxygen in a ventilator is shown in International Publication No. WO 96/24402 published Aug. 15, 1996. This system is designed for mixing gases at approximately ambient atmospheric pressure, such as oxygen and air. The mixing apparatus includes a piston disposed within a pump chamber. A flow limiting inlet controls introduction of oxygen for mixing with air, and the pressure of the oxygen is limited to an acceptable maximum pressure whereby even if the oxygen valve fails, the breathing gas will not be provided at an excessive pressure. A demand valve is alternately provided for reducing the pressure of the oxygen supplied before mixing, and a pressure sensor is also provided downstream of the demand valve for detecting failure of the demand valve to shut off the supply of the oxygen to prevent overpressurization.
It would be desirable, however, to provide a piston ventilator where oxygen can be blended with gas or air where the piston causes air to be provided to the patient in a cycle which more closely approximates the patient""s inhalation and expiration profile. A disadvantage of using a constant rate of piston movement within the cylinder to produce ventilation flow is that the flow is affected by changes in gas density and altitude, and thus requires the use of barometric pressure monitoring and input to control the piston movement rate. In turn, it would be desirable to monitor the flow of the gas breathed by the patient and provide oxygen blending based on the flow and feedback controls based on: the flow and the volume of gas in the piston cylinder to permit use of less expensive valve controls. It is also desirable to provide oxygen blending in such a piston system where oxygen enrichment can be provided for air remaining in the ventilator system downstream from the piston system after exhalation by the patient.
These and other objects are largely met by the oxygen blending system of the present invention. That is to say, the oxygen blending system hereof uses a piston ventilator which is sufficiently compact for home use, controls the operation of the piston to provide oxygen blending in a non-constant flow rate of breathing gas to the patient, and provides enriched oxygen to the patient side of the ventilator circuit, i.e. downstream from the piston, during piston retraction to optimize the oxygen content of all of the air inhaled by the user.
The piston ventilator of the present invention broadly includes an oxygen blending module, a primary piston-driven pressurization assembly for providing positive pressure flow of breathable gas to the patient, a secondary make-up gas module, a controller, an exhalation control system, and a patient circuit for delivering air to the patient for inhalation and exhausting exhaled air. The oxygen blending module includes a connection to a source of pressurized oxygen, a first control valve which regulates the flow of oxygen to the piston, and a flow sensor for monitoring the flow of oxygen to the piston. In addition, the oxygen blending module includes a second control valve for regulating the amount of oxygen delivered to the patient circuit to enrich the gas remaining in the patient circuit during the retraction stroke of the piston. The valves are current sensitive orifice valves responsive to signals from the controller, which preferably includes a microprocessor. The flow sensor is operatively connected to the controller to provide signals corresponding to the flow of oxygen to the primary piston-driven pressurization system.
The primary piston-driven pressurization system receives oxygen from the oxygen blending module and air or another breathable gas and is operated by a motor, gear drive and cam arm to provide a flow of blended gas therefrom. That is to say, the primary system receives a low volumetric flow of blended gas at the beginning of its retracting intake stroke building to a maximum volumetric flow of blended gas during the intermediate portion of its retracting stroke and then reducing to a low volumetric flow of air at the end of its retracting stroke before beginning the protracting stroke. When relatively large volumes of breathable gas are delivered during protraction of the piston within the cylinder, the primary system delivers a low volumetric flow at the beginning of protraction building to a maximum volumetric flow of blended gas during the intermediate portion of its protracting stroke and then reducing to a low volumetric flow of gas at the end of its protracting stroke. When the volume of gas to be provided to the patient circuit is relatively low, the flow will increase abruptly and then reduce to a smaller flow at the end of the protracting stroke. Alternatively, the movement of the motor and thus the piston may be controlled to expel the blended gas more constantly to provide a flow to the patient of sustained and substantially constant pressure.
Because the increase of the volume above the piston in the cylinder is non-linear but rather sinusoidal during intake and blending, the flow of the oxygen into the cylinder is similarly non-linear. The motor driving the piston preferably provides a virtually continuously updated signal to the controller corresponding to the position of the piston, which permits the microprocessor to calculate by integration the volume of gas in the cylinder during the retraction stroke and similarly the volume of added oxygen which should have passed by the flow sensor and should be present in the cylinder. By continuously updating the comparison between the calculated amount of oxygen in the cylinder with the actual amount of oxygen delivered to the cylinder during the retraction stroke, the controller can substantially continuously signal the first control valve to open or close to provide the desired amount of oxygen enrichment to the cylinder. Preferably, the motor is a motor capable of bi-directional movement so that an adjustable end-of-travel sensor can be provided to initialize the position of the motor with the controller and thereafter acts as a safety limiter, with the controller providing a signal to the motor to reverse direction between protraction and retraction and thereby accommodate users of different lung capacities.
The secondary make-up module uses a low-pressure blower to provide make-up air or other breathable gas to the system to compensate for leakage, in particular the leakage from around tracheal tubes inserted into the patient""s windpipe or mouth. The controller provides a speed control signal to the blower to maintain the appropriate amount of pressure in the patient circuit based upon the amount of flow out of the patient circuit. The controller senses the amount of flow out of the patient circuit, which in turn operates an oxygen valve to maintain a satisfactory oxygen enriched gas in the patient circuit. The speed control signal, together with the amount of total oxygen specified to the controller, operates the second oxygen control valve to permit greater or smaller flows of oxygen to flow to the patient side of the ventilator and thus into the patient circuit for maintaining satisfactory oxygen enrichment in the patient circuit during retraction of the piston in the cylinder preparatory to initiation of inhalation. The primary flow sensor provides a signal corresponding to the volume of mixed gas leakage in the patient circuit (VT). The volume of oxygen (O2) introduced is known by introducing O2 gas of known pressure upstream of a known orifice size for a specific period of time, yielding an oxygen volume (VO2).
The concentration of oxygen in the make-up gas is then known by the equation:
VT=VAIR+VO2
O2%=79(VO2xe2x88x92VT)+21
where VT is obtained by a measurement by the primary flow sensor and VO2 is known by how long the primary oxygen control valve is open, the orifice size, and the upstream pressure.
The exhalation control system is positioned on the patient side of the ventilator for connection to the patient circuit which is connected to the ventilator and includes a flow sensor for monitoring the flow of breathable gas to the patient, pressure sensors for detecting the pressure in the patient circuit during inhalation and exhalation, and a positive end expiratory pressure control valve. The patient circuit is attached to the ventilator for connection to the exhalation control system and includes and a pneumatically controlled exhalation valve. The positive end expiratory pressure (PEEP) control valve regulates the amount of gas delivered to a diaphragm, preferably an inflatable balloon diaphragm, in the exhalation valve by selectively venting gas from the diaphragm in the exhalation control system prior to delivery to the patient circuit. By such venting, a pneumatic signal is provided through a signal conduit as the pressure on the diaphragm increases relative to the pressure in the patient circuit and the resistance of the exhalation valve to the flow of gases from the patient circuit increases. Excess gas from the diaphragm is exhausted through the PEEP control valve to the atmosphere. An increase in the pressure on the diaphragm thereby increases the amount of PEEP, that is, the pressure remaining in the patient""s airway after exhalation, which in turn resists collapse of the patient""s lungs and enhances the rapidity with which the patient may begin effective inhalation of the next breath.
As a result, the movement of the piston directly corresponds to the inhalation and exhalation of the patient pneumatically connected to the ventilator, with the desired amount of oxygen enrichment provided to aid the patient""s respiration. The enrichment is provided notwithstanding characteristics of the intake flow of breathable gas into the piston and cylinder assembly whether it be linear or sinusoidal because the amount of oxygen added to the cylinder is constantly monitored and controlled in a closed loop system. Furthermore, the make-up air or other gas provided to the patient is oxygen enriched, and provided in connection with a positive end expiratory pressure control to enhance the rapidity with which the next breath may be inhaled. These and other advantages will be readily apparent to those skilled in the art with reference to the drawings and description set forth below.